Boxoffice-July.1999

\
SPECIAL REPORT: Sound
SETTING SUBWOOFER LEVEL
for only four films, all the bass was delivby
John F. Allen
Although
stereo motion picture
sound systems have employed
subwoofers for over two decades,
a standard method for setting their playback
levels has only recently been agreed
to. This may seem surprising unless one
remembers how standards and recommended
practices are arrived at in the
first place. Rather than an individual or
group sitting around a table and dictating
a standard or a specific practice that
all must follow, standards committees
typically recognize established industry
practices and agree on how to describe
them. The first
use of many such practices
often originates with various manufacturers.
But not until it is acknowledged
that such practices are in general
use, or misuse as the case may be, are they
considered by a standards committee.
The committee that oversees audio
standards and practices for motion pictures
is the Committee on Audio
Recording and Reproduction Technology
of the Society of Motion Picture
and Television Engineers (SMPTE). For
some years now, this group has been
actively updating the various standards
and recommended practices while discarding
those that have become obsolete.
(It's amazing how hard some of
these things die.) There is no previous
industry-wide practice regarding subwoofer
levels to update, so a new recommended
practice has been adopted.
However, because DTS, Dolby and Sony
have each specified a different method
for setting subwoofer levels, it wasn't as
simple as one might think.
The subwoofer channel was added to
address the need for bass in movie theatres.
In the mid-1970s, with the exception
of the Sennsurround process used
ered by the main full-range screen
speakers. The largest screen speaker typically
in use then was the Altec Lansing
A-4. Large as it was, the A-4's woofer
had a falling frequency response. The
lower the bass frequency, the less the
speaker's output. To a degree, this is true
for every woofer ever made. In the case
of most theatre speakers, the fall-off
began anywhere from 80 to 200 hertz.
With a few exceptions, this is pretty
much the capability of the woofers in
many of today's screen speakers. In
addition, rooms as large as movie theatres
(even small ones) simply must have
more bass output from the sound system
in order for the bass to be in balance
with the middle and higher frequencies.
This especially true for sound systems
required to reproduce explosions.
Originally, Dolby Laboratories specified
that the subwoofers be set at 89 to
91 dB in 70mm theatres with pink noise
playing and measured in the center of
the theatre. It soon became apparent
that this approach resulted in widely
varying subwoofer levels from theatre to
theatre when playing actual program
material. This is due to many factors,
such as room reverberation and size.
Subwoofer systems in two different theatres
could measure the same with pink
noise, but be as much as 6 dB apart when
a film played. Consequently, the only reliable
way to set subwoofer levels was by
ear, along with familiar material. Another
measurement method was needed.
Dolby's answer was to use a Real-
Time-Analyzer (RTA) rather than a
sound pressure level meter to measure
the subwoofer level. The idea was that
the subwoofer frequencies should be set
to appear 10 dB above those of the center
channel, as seen on the analyzer. (See
Figure 2.) Personally, I was skeptical
about this approach. However, though I
was unsure of this method, I did begin
an extensive study to see if indeed the
subwoofer's frequencies measured 10 dB
above those of the center channel, when
the subwoofer levels were correct and
sounded the same in different theatres.
Sure enough, they did. Yet the pink
noise-based subwoofer readings in these
different theatres varied from 85 to 92
dB when measured with an SPL meter.
When DTS and Sony initially
introduced
their digital processors, they specified
that their subwoofer levels should
be measured and adjusted using an SPL
meter, just as Dolby had originally done.
Recently, both Sony and DTS
agreed with the Real-Time-Anal;<
approach, as they also found it tc
more reliable. Consequently, the SMI
has issued a recommended prac|
RP200, which is described below.
First,
I have a suggestion: U:
there is a very unlikely au
problem with the frequency
sponse of a subwoofer, the subwoi
channel should not be equalized
measurement systems used by th
technicians are simply too unreliabli
be equalizing a speaker playing
these lowest frequencies. Subwoofer
not need to be equalized, even thoug!
the measurement systems might seem t<
|
say that they do. Beyond limiting th
|
bandwidth of the subwoofer channel t(
frequencies below 80 to 100 hertz, I hav
never found the need to equalize a sub
woofer in twenty years.
Each of the DTS, Dolby and Sony cin
ema processors employ a different mean
for equalizing subwoofers. The best way
have found to set the subwoofer equaliza
tion with the various Dolby units is to ust
the adjustments to simply minimize th(
frequencies above 80 to 100 hertz. Th(
original Sony DFP-2000 processor a;
well as the DTS DTS-6 and DTS-6E
players have fixed or selectable subwoofei
filters that allow only the frequencies
below around 80 hertz to pass. The newei
analog-digital processors from these
companies employ optional subwoofer
equalizers. In the Sony DFP-3000, for
instance, one can set the subwoofer filteri
at 100 hertz (as is done with the DFP-i
2000) and leave the subwoofer equaliza-;
tion adjustments flat.
Rather than relying on a Soundj
Pressure Level meter, subwoofer levels|
should be established as follows:
i
1 After the screen channels are equal-l
ized and levels are set, play pink noisej
through the center channel. Observe the
RTA display and note the level of the fre-l
quencies between 100 and 2000 hertz.'
(See Figure I.) This level becomes an;
imaginary reference line for setting bothj
the optical and digital subwoofer levels.)
Turn off the center-channel pink noise, i
2. Turn on the pink noise in the sub-1
woofer channel. Place the processor in;
the digital format. Adjust the digital]
subwoofer so that the subwoofer fre-j
quencies are 10 dB above the (imagi-i
nary) reference line. (See Figure 2.
34 BOXOFFICE